The prior art includes a wide variety of devices which generally fall into the broad category of pressure transducers. As one can ascertain, certain of these devices employ piezoresistive elements. These elements exhibit a change in resistance as a function of an applied force or pressure. Another class of pressure transducers employ a variation of capacity to determine the magnitude of an applied force. These devices operate to vary effective capacity between a movable plate and a stationary plate. The movable plate can also be a flexible diagram which will deflect upon application of a suitable force by an amount proportional to the force. The motion of the plate serves to vary the effective distance between the movable plate and the fixed plate. As is well known, the distance between two parallel plates determines the magnitude of the effective capacitance. Capacitive transducers are usually difficult to fabricate and are not necessarily compatible with conventional integrated circuit techniques.
Reference is made to U.S. Pat. No. 3,748,571 entitled, “Pressure Sensitive Transducers Employing Capacitive and Resistive Variations” which issued on Jul. 4, 1973 to A. D. Kurtz, the inventor herein, and is assigned to the assignee herein. Reference is also made to U.S. Pat. No. 4,814,845 entitled, “Capacitive Transducers Employing High Conductivity Diffused Regions”. This patent issued on Mar. 21, 1989 to A.D. Kurtz and is assigned to the assignee herein. As one can ascertain from U.S. Pat. No. 4,814,845, it shows a semiconductor capacitive transducer which is fabricated by utilizing layers in a sacrificial wafer. By processing the sacrificial wafer, one can obtain a P+ region which is indicative of one plate of the capacitor. A carrier wafer has a dielectric layer on a top surface which includes a layer of glass. The sacrificial wafer, after being subject to diffusion of highly doped semiconductor materials, contains a peripheral flange and a capacitive region which is indicative of a capacitive plate. By utilizing selective etching, one can thus form a capacitive structure on a sacrificial wafer which is bonded to the carrier wafer by means of an electrostatic bond. This device is, in fact, compatible with semiconductor processing techniques.
As one can ascertain, such capacitive devices can be connected in arrangements for use in conjunction with an inductor to provide frequency outputs which are proportional to the force or applied stress. It is also indicated that one can utilize such capacitive transducers together with inductors to also obtain resonant circuits which frequency would change upon application of a force to vary the capacitive component of the resonant circuit.
The present invention relates to the fabrication of an improved capacitive inductive pressure transducer which is fabricated by MEMS techniques and essentially, provides an ultra-miniature high temperature device.